This invention relates to ceramic materials and to methods for making and using such materials. More specifically, this invention relates to ceramic materials having surprising and unexpected diamagnetic moments. Yet even more specifically, this invention relates to ceramic materials which exhibit extremely low resistance to the passage of electrical current at very high temperatures and at atmospheric pressure. Essentially, this invention relates to very high temperature superconducting compositions and to methods of making and using same.
Materials which exhibit the phenomenon of total diamagnetism at non-cryogenic temperatures and easily attainable pressures have been the subject of considerable research interest for a significant period of time. That research, which has been referred to as xe2x80x9chigh temperature superconductorxe2x80x9d research, has focused upon the resistive and diamagnetic properties of the materials of interest at 23 degrees Kelvin (K) and above (i.e., 23 degrees above absolute zero). In other words, xe2x80x9chigh temperaturexe2x80x9d, as that term has been used in the past, referred to temperatures only attainable under extremely cold, namely, cryogenic conditions.
A major advance in the world of superconducting compositions is described in the popular novel, The Breakthrough the Race for the Superconductor written by Robert M. Hazen. The Breakthrough describes the massive scientific effort that was generated when, in 1986, George Bednorz and Alex Muller reported synthesis and testing of a Baxe2x80x94Laxe2x80x94Cuxe2x80x94O composition which exhibited superconductive behavior at 30 degrees K (xe2x80x9cPossible High Tc Superconductivity in the Baxe2x80x94Laxe2x80x94Cuxe2x80x94O Systemxe2x80x9d in Zeitschrift fur Physik, November, 1986). For their efforts, Bednorz and Muller received the Nobel Prize in physics in 1987. Having shown that the perceived 23 degree K barrier could be broken, the work of Bednorz and Muller spurred the research efforts of many other researchers at several other research laboratories.
Subsequent to the work of Bednorz and Muller, several other researchers synthesized and tested compositions which exhibit superconductive characteristics in the xe2x80x9chigh temperaturexe2x80x9d range as that was understood at the time. In other words, those materials exhibited superconductive characteristics at critical temperatures, Tc, of greater than 23 K. As discussed in Breakthrough, Paul Chu (Paul Ching-Wu) and his colleagues synthesized and tested an yttrium, barium, copper system (subsequently known as xe2x80x9c1-2-3xe2x80x9d reflecting the atomic ratios of the substances) which exhibited Tc of greater than 70 degrees K. This was an important breakthrough because it permitted the creation of superconducting behavior at about the boiling temperature of liquid nitrogen, i.e., 77 degrees K. This achievement (which was characterized as the equivalent of breaking the 4-minute mile in distance racing) permitted research and applications work to proceed using inexpensive, easily handled cryogenic fluid, liquid nitrogen. Research at liquid nitrogen temperatures and above is one of the factors which permitted the phenomenon of superconductivity to emerge from the stage of laboratory curiosity and to begin its journey to practical application.
In their efforts to understand superconductivity better and thereby, potentially, to increase Tc to even more useful ranges, Chu and others began evaluating superconductor candidates at elevated pressures. At pressures in excess of 10,000 atmospheres (1xc3x97107 k-Pascals), shifts in Tc, either up or down, were regularly observed. As recently as Feb. 10, 1993 Chu has reported (in New Scientist, p. 14) a Tc of 160 degrees K at a pressure of more than 150,000 atmospheres with respect to a HgBa2Ca2Cu3 material. Mercury superconductors were apparently first identified by the CNRS French national research organization in early 1993, c.f. New Scientist, Science, 27 March 1993.
In the first instance, even though considerably more convenient than early superconductor work, cryogenic equipment needed to handle liquid nitrogen at 77 K is expensive, cumbersome, and potentially dangerous. This is particularly true of commercial applications of superconductor materials where cumbersome, expensive, and inconvenient cryogenic equipment is needed to cool the superconductor material to create the desired superconductivity. Clearly, applications which required super atmospheric pressures to reveal the desired elevated Tc would additionally complicate utilization of superconductor compositions as well as increase the expense and inconvenience of doing so.
Also of note is the fact that most materials which have been shown to exhibit superconductivity are oxides. Table 1 entitled xe2x80x9cLHigh Temperature Superconductorsxe2x80x9d at page 12-76 of the 1993-1994 edition of the CRC Handbook of Chemistry and Physics lists approximately 30 materials, only one of which (Rb2CsC60) is not an oxide. (Table 1 indicates preparation in November, 1992.) That non-oxide composition employs carbon, which itself likely has characteristics similar to those exhibited by the metals which comprise the rest of the materials. The highest Tc listed in the above-mentioned Table 1 is 128 degrees Kelvin for a Tl2Ca2Ba2Cu3O10 system.
There is, therefore, a critical need for a material which exhibits superconductivity at non-cryogenic temperatures (i.e., above about 273 degrees K) and substantially atmospheric pressure. This invention describes such a material.
The present invention is a ceramic composition which exhibits diamagnetic behavior and electrical resistivity, indicating that it is a superconductor. A material of this invention exhibits these properties at a wide range of temperatures and pressures, most notably, at room temperature and atmospheric pressure. In its preferred practice, this invention is a composition that exhibits all the attributes of superconductivity at room temperature and atmospheric pressure. Reduced to its essentials, this invention is the next revolution in superconductivity. It begins the era of what will hereinafter be defined as very High Temperature Superconductors (VHTS).
Briefly, the present invention, in one aspect, is a method for conducting electricity utilizing a superconducting ceramic material at a temperature which is greater than 193 degrees K, a pressure which is less than 150,000 atmospheres. Generally, such materials will have a resistivity of less than 1xc3x9710xe2x88x928 ohm-meters. For all purposes herein, superconductivity at a temperature in excess of 193 degrees K will be considered VHTS. Preferably, a method of this invention involves conducting electricity at a temperature in excess of about 270 degrees K. More preferably, a method of this invention involves conducting electricity at a temperature in the range of about 273 degrees K to about 373 degrees K, without cryogenic cooling, at a pressure in the range of 0.1 to about 10 atmospheres, under conditions of superconductivity. Most preferably, the present method involves the method of conducting electricity at about room temperature, i.e., about 300 degrees K, atmospheric pressure, under conditions of superconductivity.
In another aspect, the present invention is a ceramic material which exhibits superconductor characteristics within the above-defined pressure and temperature ranges, the material comprising:
MMxe2x80x22Mxe2x80x33Zaxe2x88x92b
wherein:
M is a metal selected from the group consisting of the elements Y, La, Tl, and mixtures thereof;
Mxe2x80x2 is a metal selected from the group consisting of Ba, Ca, and mixtures thereof;
Mxe2x80x3 is a metal selected from the group consisting of Ag, Cu, Au, Rb, Cs, K, and mixtures thereof;
Z is selected from the group consisting of S, Te, and Se; wherein
a is an integer having a value between 2 and 10, preferably between 5 and 8, and
b is in the range of about 0 to about 0.5.
In a preferred practice of this invention, M is Y (i.e., yttrium), Mxe2x80x2 is Ba, Mxe2x80x3 is either Cu or Ag, Z is either Se or Te, a has a value of about 7 and b is in the range of about 0 to about 0.5.
In yet a further preferred practice of this invention, M is Y, Mxe2x80x2 is Ba, Mxe2x80x3 is either Cu or Ag, and Z is Se.
xe2x80x9cSuperconductivexe2x80x9d or xe2x80x9csuperconductingxe2x80x9d as the term is used herein is defined to mean, having the property of passing electronic currents without an observable energy interchange occurring between current carriers (e.g., electrons) and lattice structures. Superconduction is observed when the flow of current fails to generate an observable accompanying voltage gradient. Currents passing through a superconductor will persist substantially indefinitely. Therefore, a nondegrading magnetic field will be observed due to this current. A superconductive material will be substantially diamagnetic.